Variable stage charge pump

ABSTRACT

A variable stage charge pump for a flash memory device is described. The variable stage charge pump includes a first charge pump and a second charge pump. A first switch couples an output of the first charge pump to an input of the second charge pump. A second switch couples an input of the first charge pump to the input of the second charge pump. The first and second charge pumps are series-coupled to a common output node when the first switch is in a first position and the second switch is in a second position, wherein the first and second charge pumps are parallel-coupled to the common output node when the first switch is in the second position and the second switch is in the first position.

This is a divisional of application Ser. No. 08/537,233, filed Sep. 29,1995, now U.S. Pat. No. 5,602,794.

FIELD OF THE INVENTION

This invention relates to the field of charge pumps. More particularly,this invention relates to providing a charge pump having a variablenumber as opposed to a fixed number of stages according to charge pumppower supply input levels and desired charge pump output levels.

BACKGROUND OF THE INVENTION

A current trend in the electronics industry is to reduce the powerrequirements of integrated circuitry. In order to reduce powerconsumption, integrated circuits are being designed to use lower voltagesupply levels such as 3.3 volts instead of 5 volts, for example.

Many operations, however, require voltages greater than that provided bythe lower voltage power supplies. For example, flashelectrically-erasable programmable read only memory (flash EEPROM)requires approximately 12 volts for erase and programming operations.

Charge pump technology permits the generation of voltages having amagnitude greater than that of the power supply. Through the use ofcharge pump circuitry 12 volts can be generated from 3.3 volts by usingone or more charge pump stages.

In the electronics field, the practitioner often needs to generate anumber of voltages from one available power supply voltage. For example,computer memory circuitry might require one voltage for reading, anothervoltage for writing, and yet a third voltage for erasing the memory.Alternatively, different components of the computer system (such asdifferent types of memory) may have power supply requirements thatdiffer from each other and from the available power supply voltage.

Another factor to consider when designing integrated circuits is thateven though a trend might indicate a general acceptance of a progressiontowards another power supply level, there may be an established base ofcircuitry that will not readily change. For example, although a currenttrend may be moving towards a 3.3 volt power supply system, there may bean established base of hardware dependent upon a 5 volt power supply.

One prior art method used to generate multiple higher voltage levelsfrom a single power supply voltage is to use a number of charge pumpcircuits each having a fixed number of stages. One disadvantage of thismethod is that each charge pump circuit requires a dedicated space inthe integrated circuit. Another disadvantage is that the integratedcircuit might not be able to be interchangeably used with both a 3.3volt power supply and a 5 volt power supply without additional externalcircuitry. For example, a manufacturer might find it advantageous toprovide a single flash memory product that could accommodate differentpower supplies.

Another prior art method used to generate multiple higher voltage levelsfrom a single power supply voltage is to use one charge pump circuitthat used a fixed number of stages to generate a voltage level. Othervoltage levels are provided by using voltage divider networks inconjunction with the single charge pump. One disadvantage of this methodis that power is wasted on the voltage divider networks. Anotherdisadvantage is that the fixed stage method typically sacrificesperformance at one power supply voltage to accommodate another powersupply voltage.

SUMMARY AND OBJECTS OF THE INVENTION

A variable stage charge pump is described. The variable stage chargepump includes first and second charge pumps. A first switch couples anoutput of the first charge pump to an input of the second charge pump. Asecond switch couples an input of the first charge pump to the input ofthe second charge pump. The first and second charge pumps areseries-coupled to a common output node when the first switch is in afirst position and the second switch is in a second position. The firstand second charge pumps are parallel-coupled to the common output nodewhen the first switch is in the second position and the second switch isin the first position.

One object is to provide a charge pump that can accommodate differentoutput levels for a given charge pump power supply input level.

Another object is to provide a charge pump that can accommodate a givenoutput level for different charge pump power supply input levels.

Other objects, features, and advantages of the present invention will beapparent from the accompanying drawings and from the detaileddescription that follows below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements and in which:

FIG. 1 illustrates a variable stage charge pump.

FIG. 2 illustrates circuitry including variable stage charge pumps forsupplying power to a flash memory array.

FIG. 3 illustrates a variable stage charge pump coupled to a flashmemory array.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of a variable stage charge pump.Variable stage charge pump 100 includes a number of "rungs" (e.g., 162,164). These rungs are coupled between a common input bus (160) andcommon output node 150. Each rung includes a charge pump. For example,rung 162 includes charge pump 110 and rung 164 includes charge pump 120.Each charge pump includes one or more fixed stages. In FIG. 1, the firstcharge pump (110) is made up of series-coupled charge pump stages 112and 114. The second charge pump (120) is made up of series-coupledcharge pump stages 122 and 124. A power supply voltage, Vpp, providesthe supply voltage to common input bus 160.

A first switch (130) controls whether the output of the first chargepump is provided as an input to the second charge pump. A second switch(131) is used to couple the input of the second charge pump to Vpp.Switches 130 and 131 control whether charge pumps 110 and 120 areseries-coupled or parallel-coupled. The variable stage charge pump 100may also be identified by the expression "X/Y stage pump". In such acase, the "X" refers to the number of rungs (and therefore the maximumnumber of fixed stage charge pumps) that may be coupled in series. The"Y" indicates the number of stages for each fixed stage charge pump.Thus "X times Y" indicates the maximum number of stages that can becoupled in series between the common input bus and the common outputnode. In this example there are 2 rungs. Each rung has a charge pumpwith 2 stages. Thus, variable stage charge pump 100 might also bereferred to as a "2/2 stage pump". The 2/2 stage charge pump can coupleall four stages in series or two series-coupled stages can be parallelcoupled to the other two series-coupled stages.

There are four possible position combinations for switches 130 and 131(two positions times two switches). The phrase "on" when referring toswitches 130 and 131 means that the switches provide a current path. Thephrase "off" when referring to switches 130 and 131 means that theswitches are in an open-circuit state. In other words, the switches donot permit substantial current to flow through them. In one embodiment,switches 130 and 131 are implemented from metal oxide semiconductorfield effect transistors (MOSFETs). In order to ensure that the greatestrange of voltages can be passed by MOSFETs 130 and 131, V_(OUT) fromcommon node 150 is used as the control voltage to the MOSFET gates.

By choosing a different one of the four position combinations, thenumber of stages and the coupling of the charge pump stages to eachother can be varied. The table below indicates the results of the fourcombinations:

                  TABLE 1                                                         ______________________________________                                        Switch   Switch  Charge Pump   Charge Pump                                    130      131     110           120                                            ______________________________________                                        OFF      OFF     ON            OFF                                            OFF      ON      ON            parallel-coupled to                                                           charge pump 110                                ON       OFF     ON            series-coupled to                                                             charge pump 110                                ON       ON      N/A           N/A                                            ______________________________________                                    

From the above table, if both switches are off, charge pump 110 will bethe only charge pump providing voltage to node 150. Thus, because chargepump 110 is a two stage charge pump, this configuration of switches 130and 131 will effectively result in a two stage charge pump. Thisconfiguration might be used in a power saving mode if charge pump 100 isotherwise providing sufficient current and voltage output levels.

If switch 130 is off and switch 131 is on, charge pump 120 will becoupled so that charge pumps 110 and 120 are coupled in parallel tocommon output node 150. Because both 110 and 120 are two stage chargepumps, this means that charge pump 100 will effectively be a four stagecharge pump having two sets of two stages coupled in parallel. In thisconfiguration, charge pump 100 will provide approximately the sameoutput voltage at node 150. Because 110 and 120 are parallel-coupled,charge pump 100 will be able to provide approximately twice the amountof current as was the case when only charge pump 110 was operating.

If switch 130 is on and switch 131 is off, charge pump 120 will becoupled so that charge pumps 110 and 120 are series-coupled. In otherwords, the input voltage for charge pump 120 will be provided from theoutput of charge pump 110. Because both 110 and 120 are two stage chargepumps, this means that charge pump 100 will effectively be a four stagecharge pump with all four stages serially coupled. In thisconfiguration, charge pump 100 will provide approximately twice theoutput voltage at node 150 as the previous two cases. However, chargepump 100 might only be able to provide approximately 1/2 the current aswas the case when charge pumps 110 and 120 were parallel-coupled.

Table 1 indicates an "N/A" for the fourth configuration. "N/A" indicatesthat this configuration is not applicable because it should not be used.If both switches are on, the first charge pump will be shorted outbecause its output is coupled to its input and the supply voltage. Suchoperation may be harmful to the charge pump circuitry in this embodimentand should be avoided.

FIG. 1 also illustrates n-channel MOSFETs 140 and 142. Transistors 140and 142 are connected in diode fashion between the outputs of chargepumps 110 and 120, respectively, and output node 150.

This prevents the charge pumps from shorting each other out whenseries-coupled. For example, if switch 130 is on and switch 131 is off,charge pump 120 would be shorted out without diode-connected transistor140. If transistor 140 were not in place, node 144 would be connected tonode 150. Node 144 would also be connected to the input of charge pump120. Because the output of 120 is coupled to node 150, this would resultin the output of charge pump 120 being coupled to the input of chargepump 120. Such a configuration would render charge pump 120 inoperativeand thus the series-coupled configuration could not otherwise beobtained.

With transistor 140 in place, if the voltage at node 150 is greater thanthe voltage at node 144, no current can flow through transistor 140.This is the situation when the charge pumps are series-coupled. Whencharge pumps 110 and 120 are series-coupled, the voltage at node 146will be greater than the voltage at node 144. If the voltage at node 146exceeds the voltage at node 150 by the threshold voltage of transistor142, transistor 142 will conduct to provide the voltage available atnode 146 to node 150 (less the threshold voltage of transistor 142). Aslong as the voltage at node 144 less the threshold voltage of transistor140 does not exceed the current voltage on node 150, transistor 140 willnot conduct and will thus ensure stability of the series-coupledconfiguration for the charge pumps.

Using low threshold voltage MOSFETs helps to reduce the voltage dropacross transistors 140 and 142 and thus a higher percentage of thevoltage supplied by the charge pump stages will reach output node 150.Typical threshold voltages for low threshold voltage MOSFETs are usuallyless than 1 volt and typically range from 0.4 to 0.7 volts.

In one alternative embodiment, charge pump 100 is a negative charge pumpproviding voltages less than zero from Vpp. In such a case pump stages112, 114, 122, and 124 are negative charge pumps. Vpp might be systemground. In order to function properly, transistors 140 and 142 arep-channel MOSFETs in this embodiment.

In another alternative embodiment, the variable stage charge pump mightbe constructed with n rungs of charge pumps, each charge pump having ystages. Thus the variable stage pump is an "n/y" stage pump according tothe definition above. By selecting an appropriate subset of the firstand second switches, the n charge pumps may be divided into m sets of pcharge pumps. Each set is now an "p/y" variable stage charge pump. Now avariety of combinations can be achieved because the sets may be series-or parallel- coupled. Furthermore, the rungs within each set may beseries- or parallel-coupled. For example, assume a variable stage chargepump having 12 rungs. Each rung includes one fixed stage charge pump.Each fixed stage charge pump includes 2 series-coupled stages. This is a12/2 stage pump. Thus 12 pumps having two stages may beparallel-coupled, or all 24 stages may be series-coupled.

The 12 rungs may be grouped, however, into 4 sets of three rungs. Thuseach set is a 3/2 stage pump. This means that within the defined setthere are two possible combinations. Either all six stages may becoupled in series or three sets of two stages may be coupled inparallel. Furthermore, the four sets may be parallel-coupled orseries-coupled. FIG. 3, for example, illustrates a variable stage chargepump with 12 rungs coupled to a flash memory array 390. Each rungincludes a charge pump having 2 stages. Each charge pump has an inputand an output. Consider the subset of the 12 charge pumps consisting ofn charge pumps comprising first charge pump 310 and m other charge pumps320-330.

The input of each of the m other charge pumps is coupled to the outputof a preceding one of the n charge pumps 310-330 by an associated firstswitch. The first switch associated with the input of the first one ofthe m other charge pumps 320 is coupled to the output of first chargepump 310. The output of each of the n charge pumps is coupled to a sameoutput node 350.

The input of each of the m other charge pumps is coupled by anassociated second switch to the input of the first charge pump 310.

The n charge pumps are all series-coupled if all the first switches arein a first position and all the second switches are in a secondposition. The n charge pumps are all parallel-coupled if all the firstswitches are in a second position and all the second switches are in afirst position.

The various combinations may be more easily seen using symbolicnomenclature. The nomenclature "S" and "P" indicates whether the rungswithin each set are coupled in series or parallel. The symbols "∥" and"-" indicate a parallel or a series coupling, respectively. Thus sevenother possible combinations using these sets are S∥S∥S∥S, P∥P∥P∥P,S-S-S-S, and P-P-P-P, S-P-P-P, S-S-P-P, and S-S-S-P. Two of thecombinations (i.e., P∥P∥P∥P, S-S-S-S) are redundant with thecombinations achievable without the step of dividing the rungs intosets. This example, however, illustrates how at least five additionalpower supply configurations can be achieved by individually controllingsubsets of rungs.

In addition to these various configurations, a subset of all the rungscan be selected to provide power. In other words, by deselecting rungsusing the appropriate associated second switches (i.e., switching themoff), less than all the rungs can be used in the variable stage chargepump. This might be used to conserve power instead of utilizing all thestages and a voltage or current divider for the powered circuitry (e.g.,memory circuitry). Voltage and current dividers tend to consume powerand should be eliminated, if possible, in order to conserve power.

The definition of the naming convention of "X/Y" now changes slightly.Previously "X" meant the total number of rungs and each rung was treatedas a fixed stage charge pump. Now, however, it is evident that byreconfiguring sets of rungs--each set is in essence a fixed stage chargepump. Therefore, "X/Y" now means X parallel coupled sets of chargepumps, each charge pump in the parallel-coupled set having Yserially-coupled stages.

Designing the variable stage charge pump with an appropriate number ofstages permits a charge pump circuit that can provide the propervoltages for the memory circuitry irrespective of the available powersupply voltage. For example, if 6 and 12 volt supplies are required fromthe variable stage charge pump, the designer can provide a control tothe switches that will switch stages in and out, or change the couplingof the stages to ensure the appropriate output voltage from the variablestage charge pump regardless of the input voltage. The control for theswitches would be a function of the available power supply voltage, Vpp,and the desired variable stage charge pump output voltage. Typically thevoltage provided by the variable stage charge pump should be greaterthan the voltage required by the circuitry to be powered because somelosses might occur if the output of the variable stage charge pump isregulated.

In one embodiment, a variable stage charge pump might be used to providethe various voltage levels required by a flash memory array. A flashmemory array is made of up memory cells including floating gate fieldeffect transistor devices. These transistors can be programmed bychanging the charge stored on a floating gate, and the condition(programmed or erased) may be detected by interrogating the cells.Different voltage requirements are needed by the flash memory cells forthe different operational modes. A variable stage charge pump (or pumps)might provide the appropriate supply voltages for each mode of operationof the flash memory array. These modes include reading, programming, anderasing.

Typically a flash memory array is subdivided into blocks and the erasemode will erase one or more blocks of memory cells. The flash memorycell is erased by removing excess charge from the floating gate. Theconventional method of erasing all the cells in a block of flash memoryrequires the application of 12 volts to the source terminals of all ofthe memory cells in the block while the drain terminals are leftfloating and the gate terminals are grounded.

Flash memory cells are programmed by placing excess charge on thefloating gate to increase the threshold voltage of the flash memorycell. Programming is typically achieved by applying approximately 11-12volts to the gate, 6-7 volts to the drain, and grounding the sourceterminal so that electrons are placed on the floating gate by hotelectron injection.

Flash memory cells are read by applying a fixed voltage to the gate ofthe flash memory cell in order to determine whether the flash memorycell is in an erased or a programmed state. This technique senses thedrain-to-source current, IDS for the flash memory cell. Reading a flashmemory cell typically requires the application of 5 volts to the gate, 1volt to the drain, and grounding the source terminal.

Thus, typical voltages required for flash memory applications include 5volts for the read mode and 6 and 12 volts for both the program anderase modes. In one embodiment, power for the flash memory device isprovided by two sources. These sources include a V_(CC) line and a Vppline. The V_(CC) line is the primary power source for the flash device.The supplemental voltage provided by supply line, Vpp, is typicallyneeded only when writing or erasing the memory because of the highervoltages needed during those operations. In one embodiment, V_(CC) isapproximately 5 volts. Vpp, however, might be 3.3, 5, or 12 volts.

Although 5 volts is a popular standard for Vpp, 3.3 volts is gaining inpopularity. The variable stage charge pumps might be incorporated intothe flash memory device in order to simplify external circuitry. Tomaximize the utility of a flash memory system, however, the variablestage charge pump must be able to generate approximately 5 volts, 9volts, and 12 volts from either a 3.3 volt or 5 volt power supply.

FIG. 2 illustrates the power supply circuitry for a flash memoryincluding variable stage charge pumps for accommodating a 5 volt, 9volt, and 12 volt voltage level from either a 3.3 volt or 5 volt powersupply. The variable stage charge pumps are designed to exceed thenominal required voltages because its output is routed to a voltageregulator. In addition, reasonable power supply tolerances typicallyrequire that the circuitry function properly when the input voltage iswithin a given percentage of the nominal value (10%, for example). Thismeans that the output voltage from the charge pumps must depend on Vppinput voltages of 3.3 volts±10% or 5 volts±10%. Assuming a 10% lower Vppthan nominal, this means that the flash power supply circuitry must beable to provide the proper voltage and current levels to the flasharrays from a Vpp of approximately 3.0 volts or 4.5 volts.

In this embodiment, two charge pumps (210,220) are used to increase theinternal node's voltages to different voltages during the read mode (5volt charge pumps), the programming mode (12 volt and 9 volt chargepumps), and the erase mode (12 volt and 9 volt charge pumps). During theread mode when V_(CC) =3.3 volts, the wordlines must be pumped to 5volts. The charge pumps are reconfigured depending upon Vpp (e.g., 3.3or 5 volts) and the mode of operation of the flash memory (e.g.,programming and erase mode).

The stage control for the variable stage charge pumps 210 and 220 isdetermined by the operational mode (i.e., erase, read, or program) andby the Vpp level. Level detectors 230, 231, and 232 determine Vpp andV_(CC) levels. 5/12 volt Vpp detector 230 is used to determine whetherVpp is at 5 volts or 12 volts. Detector 230 indicates a Vpp ofapproximately 12 volts. Variable stage charge pump 220 is a high currentvariable stage charge pump used during the read, program, and eraseoperations. Variable stage charge pump 210 is a low current variablestage charge pump used in addition to charge pump 220 only duringprogram and erase operations. The output of the Vpp detectors isprovided to circuitry 290 to select the appropriate erase andprogramming algorithms for the various Vpp levels. These algorithms mayvary depending upon Vpp and V_(CC). Circuitry 290 controls theappropriate switches (e.g., switch 274) in accordance with the erase andprogramming algorithms. These switches are used, for example, forcontrolling power to charge pumps 210 and 220 and for selecting theappropriate source of power to be supplied to the flash memory array onlines 260 and 264.

The flash memory array power is provided by lines 260, and 262, and 264.Line 260 supplies either Vpp from Vpp pad 295 or 12 volts from chargepumps 210 and 220 to the gates of the flash cells. Line 264 supplies theappropriate voltage to the drains and sources during programming anderase operations. Line 262 supplies either 5 volts from charge pump 220or V_(CC) to the flash memory device. Line 282 is used to enable ordisable V_(CC) detector 232.

Voltage controlled oscillators (VCOs) 240 and 241 are used to driveassociated charge pumps 210 and 220, respectively. V_(REF) 270 is usedto generate a reference voltage for the VCOs. The reference voltage andfeedback from the output of charge pumps are used as control voltages bythe VCOs to help control the output voltage of charge pumps 210 and 220.VCO 242 serves as a standby VCO when the flash memory is in a standbymode. The flash memory draws considerably less current in the standbymode.

The high current and the low current variable stage charge pumps arereconfigured depending upon the operational mode and the nominallydetected values of Vpp (i.e., 3.3 or 5 volts). Table 2 illustrates theconfigurations for variable stage charge pump 220 for different Vpps andoperational modes:

                  TABLE 2                                                         ______________________________________                                        High Current Variable Stage Charge Pump (220)                                        Mode                                                                   V.sub.PP program        verify erase                                          ______________________________________                                        5        18/3           18/3   18/3                                           3.3      18/3           18/3    9/6                                           ______________________________________                                    

Table 3 illustrates the configurations for variable stage charge pump210 for different Vpps and operational modes:

                  TABLE 3                                                         ______________________________________                                        Low Current Variable Stage Charge Pump (210)                                         Mode                                                                   V.sub.PP program        verify erase                                          ______________________________________                                        5        4/3            4/3    4/3                                            3.3      2/6            2/6    2/6                                            ______________________________________                                    

Thus in the embodiment illustrated in FIG. 2, when Vpp has a nominalvalue of 5 volts, variable stage charge pump 210 (the high currentcharge pump) is configured to have 18 parallel-coupled sets of stages,each set including 3 serially-coupled stages during the program, verify,and erase operational modes.

For the read mode, low current variable stage charge pump 220 is notused and can be turned off. During a read operation Vpp might be zero.Therefore if high current charge pump 220 is needed it will receive itspower from V_(CC) instead of Vpp during the read mode. If V_(CC) is lessthan 4.0 volts (as determined by the 3/5 volt detector 232), then highcurrent charge pump 220 is needed to provide 5 volts for the wordlines(i.e., gates of the flash memory cells) on line 262. In such a case,charge pump 220 is in the 18/3 configuration in this embodiment.Otherwise, V_(CC) is presumed to be sufficient and line 262 is switchedto provide the wordline voltage from VCC instead of from charge pump 220(which can be turned off because it is no longer required).

With respect to the embodiments presented above regarding flash memorycircuitry, the variable stage charge pump circuitry might bemanufactured within the same package as the flash memory array.

Alternatively, the variable stage charge pump circuitry might be locatedexternal to the flash memory array package.

In the preceding detailed description, the invention is described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

What is claimed is:
 1. A variable stage charge pump comprising:n chargepumps including a first charge pump and m other charge pumps; m firstswitches, wherein each first switch couples an input of an associatedone of the m charge pumps with an output of a preceding charge pump,wherein the first charge pump is the preceding charge pump for the firstone of the m charge pumps; m second switches, each second switchcoupling the input of an associated one of the m charge pumps and aninput of the first charge pump; wherein the n charge pumps areseries-coupled if all the first switches are in a first position and allthe second switches are in a second position, wherein the n charge pumpsare parallel-coupled if all the first switches are in a second positionand all the second switches are in a first position.
 2. The variablestage charge pump of claim 1 further comprising:a plurality ofdiode-connected metal-oxide semiconductor field-effect transistors, eachcoupled between an output of one of the n charge pumps and the commonoutput node.
 3. The variable stage charge pump of claim 1 wherein atleast one of the plurality of first and second switches furthercomprises a low threshold voltage metal-oxide semiconductor transistor.4. The variable stage charge pump of claim 1 wherein at least one of thecharge pumps includes a plurality of series-coupled stages.